Preheat the oven to 210C. Slice the onions very thinly, and slice the tomatoes as well. Lightly oil a gratin dish, put the onion slices in, and lay the tomato slices on top. Add salt, pepper and herbs, and drizzle a little olive oil over the top. Bake in the oven for 10 minutes.

Put the pork on top of the vegetables and spread it fairly lavishly with mustard. Return to the oven for 15 minutes, then remove, turn the pork over, spread more mustard on the other side, and roast for another 15 minutes. Let rest for 5 minutes before serving.